What is GPS?
GPS is the Global Positioning System. GPS uses satellite technology to enable a terrestrial terminal to determine its position on the Earth in latitude and longitude.
GPS receivers do this by measuring the signals from three or more satellites simultaneously and determining their position using the timing of these signals.
GPS operates using trilateration. Trilateration is the process of determining the position of an unknown point by measuring the lengths of the sides of an imaginary triangle between the unknown point and two or more known points.
In the GPS system, the two known points are provided by two GPS satellites. These satellites constantly transmit an identifying signal.
The GPS receiver measures the distance to each GPS satellite by measuring the time each signal took to travel between the GPS satellite and the GPS receiver.
The formula for this is:
Distance = Velocity * Time
Velocity of the GPS signal is the speed of light, approximately 300,000 Km/s.
GPS transmissions occur on a frequency of 1575.42 and 1227.60 Mhz. Both of these frequencies are within the L Band.
GPS History
GPS was originally developed for the U.S. military, but is now provided as a public service for people all over the world by the U.S. government.
Deployment of the GPS system began on 22 February 1978 with the launch of the first Block I Navstar GPS satellite. Initial Operating Capability was declared in December of 1993 with 24 operational GPS satellites in orbit. Full Operational Capability was declared in June of 1995.
GPS was developed by the U.S. military to help soldiers locate their positions. Civilian access to the GPS system was guaranteed by President Reagan as a response to the communist Chinese shooting down of Korean Airline Flight KAL-007. President Reagan hoped that GPS technology would help to prevent such a tragedy from happening again.
GPS Architecture
The GPS system is divided into three segments:
- The Space Segment
- The Control Segment
- The User Segment
The Space Segment
GPS uses twenty-one operational satellites, with an additional three satellites in orbit as redundant backup.
GPS uses NAVSTAR satellites manufactured by Rockwell International. Each NAVSTAR satellite is approximately 5 meters wide (with solar panels extended) and weighs approximately 900Kg.
GPS satellites orbit the earth at an altitude of approximately 20,200Km.
Each GPS satellite has an orbital period of 11 hours and 58 minutes. This means that each GPS satellite orbits the Earth twice each day.
These twenty-four satellites orbit in six orbital planes, or paths. This means that four GPS satellites operate in each orbital plane.
Each of these six orbital planes is spaced sixty degrees apart. All of these orbital planes are inclined fifty-five degrees from the Equator.
The Control Segment
The Master Control Station (MCS) of the GPS system is operated at Schriever Air Force Base in Colorado Springs, Colorado. The United States Air Force maintains redundant Master Control Stations in Rockville, Maryland and Sunnyvale, California.
The Air Force also maintains monitoring stations in Colorado Springs, Hawaii, The Ascension Islands, Diego Garcia, and Kwajalein.
Communications with the space segment are conducted through ground antennas in the Ascension Islands, Diego Garcia, and Kwajalein.
The User Segment
The GPS user segment is any person with a GPS receiver.
Books on the GPS
 GPS For Dummies |
GPS For Dummies gives new meaning to finding yourself. In fact, with a GPS (global positioning system) receiver, you can determine precisely where you are anywhere on this planet. If you're are planning on buying a GPS receiver or if you have one and want to get your money's worth, this guide tells you what you need to know, including:
Written by Joel McNamara, avid outdoorsman, adventure racer, search and rescue team member, and author of Secrets of Computer Espionage, GPS for Dummies is ideal for both ordinary travelers and exotic explorers. It covers a world of GPS info such as:
A companion web site has links to all kinds of free maps and resources. So explore on your computer and then explore for real! With GPS for Dummies, you'll find yourself having adventures! |
 GPS Mapping |
Learning to use a GPS is fairly straightforward; choosing which software to use is not. GPS Mapping - Make Your Own Maps guides readers through the challenging choices they face and shows them how to combine the fun of high-tech with the security of paper maps. Covering all major brands of mapping software, it also explores sources of free software and maps, how to use aerial photos and satellite imagery, and how to view backcountry locations in 3-D. GPS Mapping reduces a reader's learning curve and gets them mapping right away.
GPS Mapping will show you show to:
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 How to Do Everything with Your GPS |
Map your way around an unfamiliar city, improve your golf game, and avoid getting lost while hiking, camping, or boating. Use GPS receivers in your PDA, automobile, or cell phone for help with road navigation, outdoor adventures, games, and more. You'll even learn how to use GPS for business applications. Author Rick Broida provides insight, information, and examples on all the essentials, including real-world GPS case studies in each chapter. How to Do Everything with Your GPS will help you learn more about this remarkable technology with an increasing diversity of applications.
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 Hacking GPS |
Hacking GPS is the "user manual" that didn't come with any of the 30 million GPS receivers currently in use, showing readers how to modify, tweak, and hack their GPS to take it to new levels!
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